In electrophotography an image comprising an electrostatic field pattern, usually of non-uniform strength, (also referred to as an electrostatic latent image) is formed on an insulative surface of an electrophotographic element. The insulative surface comprises a photoconductive layer and an electrically conductive substrate. The electrostatic latent image may be formed by imagewise photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on the insulative surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with a toner composition generally containing a pigment. The toner image is then transferred onto a transfer medium such as paper and fixed thereon by heating and/or pressure. The last step involves cleaning residual toner from the electrophotographic element.
Dry toner compositions used in electrophotography are divided into one-component compositions composed of a toner generally comprising a binder resin having a colorant dispersed therein and two-component compositions composed of a toner and a carrier. Charge control agents are often melt mixed with the toner resin to control the chargeability of the toner during use. In order that toner compositions have process suitability in copying, they are required to be excellent in fluidity, anti-caking properties, fixability, chargeability, cleaning properties, etc. To improve these properties, particularly fluidity and anti-caking properties, extraparticulate inorganic fine particles are frequently added to toner compositions.
Dispersibility of the inorganic fine particles has a great influence on toner composition characteristics. Particles of poor dispersibility tend to fail to obtain desired effects of improving fluidity and anti-caking properties or tend to cause adhesion of toner particles to a photoreceptor due to insufficient cleaning, resulting in image defects such as black spots.
Particulate silica such as funded silica is used to provide good dispersibility, fluidity and anticaking properties in toner compositions. Unfortunately, fumed silica has a strong negative charge which seriously reduces the chargeability of positively chargeable toner compositions. Because the silica also effects the charge of negatively chargeable toners, it complicates the design and use of negatively chargeable toner compositions. The strong negative charge of fumed silica negatively impacts the reproducibility of both positively and negatively chargeable toner compositions by exhibiting great variations of chargeability with environmental changes between summer and winter, often resulting in background fog and insufficient density reproduction.
In an effort to overcome these problems, silica particles have been surface treated with various compounds such as aminosilanes, polyacrylates and quaternary ammonium salts to form positively chargeable silica particles. However, these particles are still charging and also are generally colored which prevents there use in certain applications. Therefore, there still is a need in the art for a suitable silica particle for use in toner compositions.
It is therefore an object of the present invention to provide an improved toner composition comprising improved silica particles.
Other objects and advantages will become apparent from the following disclosure.